RMH Q and A, Kinda

Steve posted a comment that I felt needed to be answered in a post rather than with another comment. Lets start with his comment:

Gotta question on the RMH. It looks neat, and I understand how it draws the way it does (have personal experience in just how much difference a stack makes on draw), but wouldn’t it make more sense to have a separate duct to pull in outside air for combustion? It looks like this thing will pull a BUNCH of air, and if you are pulling in the air from the room, then you are going to be replacing it with cold air from outside, yes? So why not have another duct leading outside (with some sort of damper/valve on it) that leads into the firebox to feed it?

Just wondering….

To take the first bit about “the stack causing the draw”. Actually from what I have been reading about the Rocket Mass Heater, and my personal experience with a L based Rocket Stove, it doesn’t. As in the stack doesn’t cause the draw or at least is not the primary cause. The rocket chamber in a properly insulated J or L based stove or heater produces intense combustion causing a rapid expansion of the CO2 and water vapor and “pushes” the column of exhaust out through the “flue” through the mass. As the heated gasses give up heat energy to the surrounding mass they shrink until at the exhaust exit they are only a few degrees warmer than the surrounding air and exit with very little velocity. The temperature in the heat riser ranges from a low of around 1200 degrees F to upwards of 2000 degrees F which burns all or almost all of the combustion components of the wood smoke and leaves very clean exhaust gasses to pass through the heat exchange “flue” or exhaust piping. The size of the intake opening, heat riser, and at least the first half of the exhaust piping has to be matched in size fairly closely, after that the size can be reduced but only to a certain extent or it will make it very hard to get the stove going, or produce a lot of back smoke, or both. The length of the exhaust stack/heat exchange piping also needs to be matched to the size of the cross section of the heat riser. A 6″ system (28.26 sq. in. cross section) can go 30 feet or so whereas a 8″ (50.24 sq. in. cross section) system can go in excess of 60″ prior to starting to encounter too much back pressure to flow properly. With only 12.56 sq. in. cross section, a system built with a 4 inch pipe is not very practical for a RMH but will work fine for a Pocket Rocket that has no thermal mass to heat and a much shorter stack.

So on to the section bit, “looks like this thing will draw a lot of air”. Not really, with the above answering a fair bit of this question too I think. The only air that is being drawn is if that necessary for combustion to take place so of course the more wood you pile in the more you will consume but that is limited by the amount of air that can flow into the cross sectional pipe. Kinda, sorta, maybe. 🙂 And I am gonna leave it at that until I build a RMH and test it for meself.

The outside air question is one I had thought of and so have a lot of other folks, but there are a couple of things to take into account here.

The first one being unless heated the colder outside air will have a cooling effect on the air in the burn chamber causing a slight decrease in overall efficiency and cleanliness of burn. Not much and it can be offset by bringing your incoming air through the thermal mass and warming it prior to injecting it into the burn chamber.

The second on is that unlike heating the air in a house the RMH produces more of a radiational heat which even with a lower air temp tends to make you warmer. A conventional woodburner like the Ashley that I am using now is a direct radiator, which as I sit here, means that my left side is warm (the side facing the stove) and my right side is much cooler (the side facing the north of the building and the open qhut door). But as I understand it, though never having experienced it first hand am not sure I doing a real good job of getting what I have read transmitted (translated?) into print here, the large mass (several tons of rock and cob) will warm the entire area to a much greater extent that a single high temp heat source.

However, I have considered and will probably implement an external air intake into the cob structure, and a damper to close the intake when the fire is out will be incorporated. I also plan to have a damper on the exhaust outlet so I can close that off. Hoping to figure a way to control both with one knob/lever so to make it harder to screw up lighting the system off.

Hope this makes things a bit clearer. For more information see: http://donkey32.proboards.com/index.cgi and http://www.permies.com/permaculture-forums/10.0.

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